Fast adaptation, the rapid termination of transduction current that follows a mechanical stimulus, correlates temporally with hair-bundle force production that could boost bundle displacement in response to weak stimuli. We have presented data in favor of the "release model", which proposes that Ca2+ lengthens a mechanical linkage in the transduction apparatus, reducing gating-spring tension and allowing channels to close. To determine the molecule responsible for fast adaptation, we introduced the Y61G mutation into the Myo1c genomic locus using gene targeting. We show here that both fast and slow slipping adaptation are faster in Y61G knock-in hair cells as compared to C57BL/6 control cells. Although these results show that mechanical activity of myosin-1c is required for fast adaptation in vestibular hair cells, additional controls are required to ensure that the differences in adaptation rate are not due to strain differences between the two mouse lines.